22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Space-resolved and averaged electron density in a dielectric barrier discharge
F. Kogelheide, S. Baldus, N. Bibinov, K. Stapelmann and P. Awakowicz
Institute for Electrical Engineering and Plasma Technology
Ruhr University Bochum, Universitätsstr. 150, 44780 Bochum, Germany
Abstract: Non-thermal atmospheric-pressure plasmas like the employed dielectric barrier
discharge are advantageous for various biomedical applications. Plasma characterisation is
necessary to understand the impact of the discharges on biological tissue; thus, energy
density is determined by current and voltage measurements as well as averaged and spaceresolved electron density using optical emission spectroscopy.
Keywords: DBD, optical emission spectroscopy, Abel transformation.
1. Introduction
Technical plasmas can be used therapeutically in
numerous diversified clinical applications. The possibility
of treating biological tissue as well as medical equipment
is an advantage of the usage of plasma technique in
medicine [1][2]. Non-thermal atmospheric-pressure
plasmas make a contact- and painless therapy possible. To
avoid adverse health effects for the patient, it is necessary
to know the parameters and principle of operation of the
discharge. It is possible to configure plasmas free of risk
for humans by knowledge of properties and effects.
Therefore, it is of great importance and necessity to
investigate the electrical and plasma physical properties
of the used plasma source. Due to the relevance of the
discharge in medical engineering, the energy density is
observed. The discharge is analysed by optical emission
spectroscopy. The electron density is investigated, both
averaged and space-resolved.
2. Methods
2.1. Dielectric barrier discharge
The dielectric barrier discharge (DBD) employed for
the experiments consists of a driven AlO 2 -covered copper
electrode and a voltage source. Every object with high
capacitance is suited as counter electrode, e.g. human
body. The DBD is further characterised by the possibilty
of reversing polarity. The frequency can be varied
between 75 Hz and 900 Hz and the maximum amplitude
of the voltage pulse can be adjusted between 6 kV and
19 kV. The distance beween the driven electrode and a
glass plate on the counter electrode was set to 1 mm and
the plasma was ignited using ambient air as process gas in
this gap. A more detailed description of the DBD is given
in Bibinov et al. [3].
2.2. Diagnostics
Optical emission spectroscopy (OES) is used to
determine the electron density of the DBD with varied
parameters both averaged and space-resolved. As the
DBD is ignited in ambient air, the two nitrogen rotational
bands N2 (C-B,0-0) at λ=337.1 nm and N2+ (B-X,0-0) at
λ=391.4 nm are used for characterisation. Direct electron-
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impact excitation from the ground state of nitrogen
molecule N2 (X) is considered [4]. Using the Ocean Optics
QE65000 spectrometer, emissions can be measured with a
space resolution of 1 mm when a diaphraghm is attached
to the optical fibre. However, the plasma parameters of
atmospheric-pressure plasmas strongly depend on space.
Therefore, space-resolved OES is realised applying a
CCD camera and band-pass filters as shown in figure 1.
The CCD images are calibrated and Abel transformation
is performed. In this way, space-resolved parameters can
be determined. The complete characterisation method is
described in more detail in Rajasekaran et al. [5].
Fig. 1. Measurement setup for optical emission
spectroscopy
The DBD employed for the experiments is analysed
regarding the impact of applied power on human tissue as
well.
3. Results and Discussion
The space resolution of the electron density within the
maximum voltage pulse is shown in figure 2. It can be
seen that the electron density is increased along the
electrodes. Negative polarity was applied to the driven
electrode (on top); thus, the counter electrode acts as
anode as the space resolution clarifies. The density along
the counter electrode is higher than the density along the
driven electrode. The ring-shaped increase of the density
can be explained with field enhancement due to the
geometry of the electrode.
1
5. References
[1] S. Emmert et al. Clinical Plasma Medicine, 1, 1 (2013)
[2] J. Hauser et al., Biomedical Engineering, 54, 2 (2009)
[3] N. Bibinov et al., Biomedical engineering, trends in
material science (2011) Acknowledgements.
[4] S. Keller et al., Journal of Physics D: Applied Physics,
45, 12 (2012).
[5] P. Rajasekaran et al., Journal of Physics D: Applied
Physics, 44, 48 (2011).
[6] F. P. Schmook et al., International journal of
pharmaceutics, 215, 1 (2001).
Fig. 2. Space-resolved electron density
(300 Hz, -13.5 kV)
The averaged electron densities determined using the
QE spectrometer are shown in figure 3.
2.75x1011
2.50x1011
2.25x1011
150Hz
300Hz
600Hz
ne / cm-3
2.00x1011
1.75x1011
1.50x1011
1.25x1011
1.00x1011
7.50x1010
5.00x1010
2.50x1010
0.00
-10.0
-13.5
-16.5
voltage / kV
Fig. 3. Electron densities determined for varied voltage
amplitudes and pulse frequencies
The averaged electron densities show a dependence on the
applied voltage pulse. With a raise in the applied voltage,
the electron density increases. No frequency dependance
can be recognised.
Overall, the averaged plasma parameters are compared
with the space-resolved parameters and an agreement
between them is established.
The energy densities of the plasma source do not
exceed the safety limit of 18.6 Jcm-2 for human tissue [6].
It is possible to adjust the voltage amplitude and pulse
frequency in such a way that there is no harmful impact
on human tissue during treatment.
4. Acknowledgements
This work was funded by the German Research
Foundation (DFG) with the grant PAK 816 'Plasma
Cell Interaction in Dermatology'.
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